Broadband wavelength division multiplexers (WDMs) have many applications in optical communications. For example, WDMs can be used to combine pump and signal wavelengths in an optical amplifying system. As another example, WDMs can be used to increase the long distance capacity of optical fibers by adding bands centered at different wavelengths. As an illustration, a band centered at 1.55 micrometers can be added to supplement capacity at the 1.30 micrometer band. The two bands can be used to provide bidirectional communications on a single fiber.
WDMs require components capable of combining and separating optical signals in broad wavelength bands. Such components have been of primarily two types: Mach-Zehnder interferometers and wavelength dependent directional couplers. Such devices are easily fabricated with optical waveguides using a single mask level, and they have low back reflection and high peak transmission. However, their spectral behavior is sinusoidal, and consequently they have low crosstalk over only a narrow wavelength range. Furthermore, it is very difficult to fabricate such devices with a precisely controlled wavelength of narrowband rejection. Consequently, interferometers and directional couplers are often inadequate for broadband applications, and thin film filters are needed to increase rejection ranges.
A new type of integrated optic multiplexer fabricated from waveguide arrays is described in U.S. patent application, Ser. No. 07/748,131, now U.S. Pat. No. 5,136,671, filed by C. Dragone on Aug. 21, 1991, and entitled "Improved Optical Switch, Multiplexer and Demultiplexer". These integrated devices comprise an array of waveguides disposed between a pair of couplers. They are useful for making closely spaced, narrowband channels for dense wavelength division multiplexing. However, they are not suitable for broadband multiplexers requiring low order devices. Accordingly, there is a need for an integrated optic device which can be used as a low order grating or a lens.